Two well-known medical imaging techniques in nuclear medicine are Single-Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). Both techniques generally makes use of gamma cameras, which detect gamma rays emitted from within the patient's body after the patient has been injected with a radiopharmaceutical substance, and a computer system, which processes the acquired data to generate images of internal structures or functions of the body. However, SPECT imaging is based on the detection of individual gamma rays emitted from the body, while PET imaging is based on the detection of gamma ray pairs that are emitted in coincidence in opposite directions due to electron-positron annihilations. PET imaging is therefore often referred to as "coincidence" imaging. Although past gamma camera systems have generally been either dedicated PET systems or dedicated SPECT systems, it is desirable to have a system that is capable of both SPECT and PET imaging. Many dedicated PET systems consist of a fixed array of detectors which partially or completely surround the patient. Such systems tend to be more expensive and less flexible, however, than systems which employ multiple detector "heads" that can be rotated about the patient. Therefore, it is further desirable to have dual SPECT/PET gamma camera imaging system which has multiple rotatable detector heads, rather than a fixed array of detectors.
One factor which has a significant impact on image quality in nuclear medicine is non-uniform attenuation. Non-uniform attenuation refers to the attenuation of radiation emitted from an organ of interest before the radiation can be detected, which tends to degrade image quality. One technique which has been used to correct for nonuniform attenuation is transmission scanning, in which gamma radiation from a known source is transmitted through the patient to a corresponding scintillation detector and used to form a transmission image. The transmission images provide an indication of the amount attenuation caused by various structures of the body and can thereby be used to correct for attenuation in the emission images. For purposes of performing attenuation correction on PET images, such transmission scans have commonly been implemented using coincidence transmission sources. However, for various reasons it may be desirable to perform a transmission scan for PET using a single-photon ("singles") source. See, e.g., S. K. Yu et al., "Single Photon Transmission Measurements in Positron Emission Tomography Using .sup.137 Cs, " Phys. Med. Biol., vol. 40, 1995, and R. A. deKemp, "Attenuation Correction in Positron Emission Tomography Using Single Photon Transmission Measurement," McMaster University, Hamilton, Ontario, Canada, September 1992. Coincidence events generally represent only a small fraction of the total detected events during an imaging session. Consequently, a singles transmission source may be preferable because of its higher associated countrate in comparison to a coincidence source. A higher countrate tends to provide a higher signal-to-noise ratio than a lower countrate does. Also, at higher source strengths, a singles source may result in higher counting efficiency because of the higher deadtime losses that are often associated with a coincidence source (i.e., from too much activity at the detector nearest to the source).
Thus, it is desirable to provide a dual SPECT/PET gamma camera imaging system which has multiple rotatable detectors and which provides attenuation correction for PET based on transmission scanning with a singles transmission source.